1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the zoom lens, and more particularly, to a zoom lens suitable for an imaging lens used in an image pickup apparatus such as a digital still camera, a video camera, a surveillance camera, a TV camera, or a film camera.
2. Description of the Related Art
A zoom lens used as an imaging optical system in an image pickup apparatus, such as a video camera, a surveillance camera, or a digital still camera, using a solid-state image pickup element, is required to have a compact size, a wide angle of view, a high zoom ratio, and a high optical performance. As a zoom lens that meets these requirements, a four-unit zoom lens is known which includes, in order from an object side to an image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power. Of the lens units, the second lens unit is moved for zooming, and the fourth lens unit corrects an image plane variation due to zooming.
In the four-unit zoom lens, the first lens unit and the third lens unit do not move. When the angle of view is increased, the front-lens effective diameter increases, and this makes is difficult to reduce the size of the entire zoom lens. Accordingly, a zoom lens including a first lens unit formed by five or six lenses has been proposed to lessen an increase in the front-lens effective diameter when the angle of view is increased.
U.S. Pat. No. 7,463,427 discloses an embodiment in which a first lens unit includes five lenses, in order from the object side to the image side, a negative lens, a positive lens, a negative lens, a positive lens, and a positive lens, the imaging angle of view is about 78 degrees at a wide angle end, and the zoom ratio is about five to ten.
U.S. Pat. No. 7,760,440 discloses an embodiment in which a first lens unit includes six lenses, in order from the object side to the image side, a negative lens, a positive lens, a positive lens, a negative lens, a positive lens, and a positive lens, the imaging angle of view is about 80 degrees at a wide angle end, and the zoom ratio is about ten.
To realize size reduction of the entire zoom lens while ensuring a wide angle of view, the refractive powers of the lens units that constitute the zoom lens are increased. However, if the refractive powers of the lens units are simply increased, a change in aberration due to zooming increases, and it is therefore difficult to obtain a high optical performance over the entire zoom range.
To obtain a wide angle of view while maintaining a compact size of the above-described four-unit zoom lens, it is important to properly set the refractive power and lens configuration of the first lens unit and optical arrangement of the first lens unit and the second lens unit. Particularly when the lens configuration of the first lens unit is not properly set, it is difficult to reduce the size of the entire zoom lens while ensuring a wide angle of view, and changes in aberrations due to zooming increase. This makes it difficult to obtain a high optical performance.